<head>
    <style>
body {
  background-color: #eee;
  
}
canvas {
  background-color: #eee;
  z-index: -1;
  position : absolute;
  display: block;
  margin: 0 auto;
}
</style>
</head>    
<body>
<canvas id="canvas"></canvas>
<script type="text/javascript" src="test.js"></script>
</body>